The present invention relates to a device for indicating the operating state of an electrical load and, more particularly, to a device for indicating an operating state of a load located and controlled at locations remote to a monitoring location.
Many industrial, commercial and residential environments incorporate electrical loads that are widely distributed geographically and often sited in difficult to access locations. However, the operation of these loads, for example, small fan motors or lights can be important to maintaining a safe environment or the successful completion of a process that may involve costly or hazardous equipment or materials and monitoring their operation is frequently desired.
In many applications, the operation of a remotely located electrical load is monitored and controlled from a central control. FIG. 1 illustrates a typical arrangement for controlling and monitoring the operation a remote load from a central controller. A remote control and status monitoring device 10 includes a start/stop relay 12 that is energized or otherwise controlled by digital output from the controller 14. Power from a power source 16, such as a utility transmission line, is provided by a wire 18 which is interconnected to the start/stop relay 12 of the device 10. The start/stop relay 12 is likewise interconnected to an internal sensor, namely, a current transformer 20 by a wire 22. The output of the internal sensor 20 is interconnected to a load 24 by a wire 26. The load 24 is interconnected to the source 16 by a wire 52. Accordingly, a loop for current flow is provided by wire 18, the start/stop relay 12, the wire 22, the internal sensor 20, the wire 26, and the wire 52. When the start/stop relay 12 is open, as a result of a first DIGITAL OUT signal from the controller 14, the power to the load 24 is interrupted by the open circuit. However, when the start/stop relay 12 is closed (short circuit), as a result of a second DIGITAL OUT signal from the controller 14, power is provided to the load 24. Accordingly, the controller 14 may control power to the load 24 by energizing and de-energizing the start/stop relay 12. The device 10 also provides a feedback signal to the controller 14 indicating whether current is flowing to the load. The internal sensor 20 is a current transformer that senses current flowing in wire 22 and outputs a signal to operate a solid state switch 21. The solid state switch 21 opens or closes providing a signal (DIGITAL IN) to the controller 14 in response to a flow of current to the load. The controller 14 can determine the operating condition of the load from the combination of the DIGITAL IN signal indicating whether current is flowing to the load and the DIGITAL OUT signal generated by the controller 14 indicating whether or not a closed load circuit should be expected and, therefore, whether current should be flowing to the load.
While the remote control and status monitoring device illustrated in FIG. 1 provides satisfactory monitoring of the operational status of a centrally controlled, remotely located load, in many environments it is desired to monitor the status of a remotely located load that is controlled from a location. that is remote from the monitoring location. For example, building security may desire to know whether lights controlled from a light switch on a remote floor or in a remote room are operating properly. Likewise, a person monitoring geographically distributed industrial machinery may desire to know the operating status of a cooling fan that is mounted on an machine located in a remote or inaccessible portion of an industrial facility and which is controlled by a local thermostat. While the output of a current sensor, such as the internal sensor of the remote control and status monitoring device 10, can be used to indicate whether current is flowing to a remote load, the information provide by a current sensor is insufficient to determine whether the load is malfunctioning because there is no information indicating whether the load circuit is closed and, therefore, current flow to the load should be expected. The operating state of a remotely controlled light in a remote room or a cooling fan for a remotely located machine requires knowledge of the state of the load controlling switch as well as knowledge of the current flow to the load.
What is desired, therefore, is a device for signaling the operating state of an electrical load located and controlled at locations remote to the monitoring location.